1. Field of the Invention
The present invention relates to a method and an apparatus for fusion-splicing polarization maintaining optical fibers by adjusting the polarization planes of the polarization maintaining optical fibers to be coincident with each other.
2. Description of the Related Art
A polarization maintaining optical fiber (PM-fiber) is given a birefringence property equivalently by introducing different stresses from two perpendicularly directions to a core. The typical polarization maintaining optical fibers include a PANDA fiber and a Bow-tie fiber in which stress applying members are disposed on both sides of the core. For fusion-splicing the polarization maintaining optical fibers, it is required that their polarization planes are coincided. The optical fibers maybe fusion-spliced with their polarization planes deviated at a predetermined angle, as needed. In this case, the polarization planes are once coincided, one of the optical fibers is rotated by the predetermined angle.
As a method for adjusting the polarization planes of the polarization maintaining optical fibers (hereinafter referred to as optical fibers) to be coincident with each other, an image observation method is well known of irradiating light to an optical fiber from a lateral side, and observing a transmitted optical image of the optical fiber, as disclosed in JP-A-2-196204.
FIGS. 9A-9D are views showing the method for adjusting the polarization planes to be coincident with each other through the image observation as disclosed in the above-mentioned patent. FIG. 9A is a view showing an observed state of optical fiber using an image pick-up camera. FIG. 9B is a view showing an observed optical image of optical fiber. FIG. 9C is a graph showing a luminance distribution of optical fiber. FIG. 9D is a graph showing a relation between a rotation angle of optical fiber and a difference between luminance peak distances. In FIGS. 9A-9D, reference numeral 1 denotes an optical fiber, 2 denotes a core, 3 denotes a stress applying member, 4 denotes an image pick-up camera, 5 denotes a display screen, 6 denotes an optical image of the optical fiber, 7 denotes a center of the optical fiber, 8a and 8b denote a luminance peak, and 9a and 9b denote an outer diameter position of optical fiber.
This method includes irradiating light from one lateral side of the optical fiber 1, picking up an optical image transmitted through the optical fiber on the other lateral side, using the image pick-up camera 4, and displaying the picked-up optical image as the optical fiber optical image 6 on the display screen 5. The luminance distribution of FIG. 9C is obtained from this optical fiber optical image 6, whereby the luminance peaks 8a and 8b are observed on both sides of the optical fiber center 7 owing to the refraction of lights transmitted through the stress applying members 3. Distances L and M between positions of the luminance peaks 8a, 8b and the optical fiber center 7 vary depending on the rotation positions of the stress applying members 3 for the optical fiber 1.
Accordingly, the rotation positions are adjusted so that the absolute value |Lxe2x88x92M| of a difference between the luminance peak distances for each of the optical fibers 1 to be fusion-spliced may be close to zero, thereby adjusting the polarization planes to be coincident with each other to have a good fusion-splicing. In FIG. 9A, a rotation angle of the optical fiber is indicated at 90 degrees, when observed from a direction passing through the core 2 and the center of the stress applying members 3. The absolute value |Lxe2x88x92M| at the rotation angle of 90 degrees become zero as shown in FIG. 9D. However, there are a plurality of rotation positions at which the absolute value |Lxe2x88x92M| is equal to zero when the rotation angle is from 0 to 180 degrees with respect to an observation plane of the image pick-up camera 4 as shown in FIG. 9D. Further, the absolute value |Lxe2x88x92M| is affected by the noise of the luminance distribution. Therefore, other parameters are needed to correctly acquire the predetermined rotation position (90 degrees).
As another image observation method, a technique is well known in which the positions of the luminance peaks of the optical fibers to be fusion-spliced are displayed on the display screen, and the optical fibers are adjusted so that luminance peak positions are coincident with each other (refer to JP-A-8-114720). This method is only required to align the luminance peak positions in the optical fibers to be fusion-spliced, but is not needed to have other parameters. However, this method is limited to the angles of the polarization plane at which the luminance peaks can be clearly recognized. Further, the luminance peak positions are displayed on the screen and aligned, requiring some labor and a hard task for the full automation.
The present invention has been achieved in the light of the above-mentioned problems. It is an object of the invention to provide a method and an apparatus for fusion-splicing polarization maintaining optical fibers in which a polarization plane of an optical fiber can be surely detected, and the method can be automated.
The present invention provides a method for fusion-splicing a first polarization maintaining optical fiber having a core and stress applying members which form a polarization plane and a second polarization maintaining optical fiber having a core and stress applying members which form a polarization plane, comprising: aligning an end face of the first polarization maintaining optical fiber and an end face of the second polarization maintaining optical fiber in accordance with images obtained from two different lateral directions of the first and second polarization maintaining optical fibers; adjusting the polarization planes of the first and second polarization maintaining optical fibers to be coincident with each other in accordance with a luminance distribution for each of the first and second polarization maintaining optical fibers, the luminance distribution being obtained from at least one of the images from two different lateral directions; and fusion-splicing the first and second polarization maintaining optical fibers with the thus coincided polarization planes with each other, wherein the polarization plane adjusting step includes detecting a first luminance peak closest to a first end of a bright portion of the luminance distribution and a second luminance peak closest to a second end of the bright portion, the second end being opposite to the first end, obtaining a sum of a first distance between positions of the first end and the first luminance peak and a second distance between positions of the second end and the second luminance peak, or a third distance between positions of the first and second luminance peaks, and moving at least one of the first and second polarization maintaining optical fibers such that the sum of the first and second distances becomes minimum or the third distance becomes maximum.
The present invention provides an apparatus for fusion-splicing a first polarization maintaining optical fiber having a core and stress applying members which form a polarization plane and a second polarization maintaining optical fiber having a core and stress applying members which form a polarization plane, comprising: two cameras for sensing the first and second polarization maintaining optical fibers from two different lateral directions; an image processing unit, connected to the two cameras, for obtaining a luminance distribution for each of the first and second polarization maintaining optical fibers from at least one of images from two different lateral directions sensed by the two cameras, detecting a first luminance peak closest to a first end of a bright portion of the luminance distribution and a second luminance peak closest to a second end of the bright portion, the second end being opposite to the first end, obtaining a sum of a first distance between positions of the first end and the first luminance peak and a second distance between positions of the second end and the second luminance peak, or a third distance between positions of the first and second luminance peaks; and an optical fiber holding mechanism for moving at least one of the first and second polarization maintaining optical fibers such that the sum of the first and second distances becomes minimum or the third distance becomes maximum to adjust the polarization planes of the first and second polarization maintaining optical fibers to be coincident with each other.
Here, the first end and second end of the bright portion of the luminance distribution are points where a luminance waveform of the bright portion and a background luminance threshold intersect.